Pneumatic rubber tires have treads which may be of a lug and groove configuration.
Such tires are often of a cap/base construction composed of a lugged outer tread cap rubber layer with its lugs presenting a tread running surface intended to be ground-contacting with good physical properties to promote resistance to treadwear and reduced rolling resistance for the tire itself in combination with an underlying base rubber layer to provide a cushion for the outer tread cap rubber layer.
The tread cap rubber layer is typically prepared with a relatively expensive combination of elastomers and compounding ingredients intended to promote a tire running surface with suitable resistance to tread wear and reduced rolling resistance.
During service, the lugs of the tread cap rubber layer gradually wear away until the tread cap layer of the worn tire becomes sufficiently thin that the tire should be taken out of service. At such time, a considerable amount of the relatively expensive rubber tread cap layer normally remains which is either discarded with the tire or ground away to prepare the tire for retreading.
Accordingly, motivation is present for preparing a novel cost-savings tire tread which is a departure from past practice.
For this invention, it is proposed to provide a significantly less expensive specialized transition rubber layer in a sense of containing scrap rubber and/or coal dust, where said scrap rubber is comprised of at least one of partially depolymerized cured rubber and particulate pre-cured rubber, which underlies, said tread cap layer and is therefore exclusive of said outer tread cap rubber layer and exclusive of said base rubber layer, if used.
In practice, the outer tread rubber cap layer is typically of a rubber composition containing reinforcing filler comprised of rubber reinforcing carbon black, precipitated silica or a combination of rubber reinforcing carbon black and precipitated silica. A major function of the tread cap layer is typically to promote a reduction in rolling resistance, promote traction for the tire tread as well as to promote resistance to tread wear.
The optional tread base rubber layer is typically composed of a softer and cooler running rubber composition, as compared to the rubber composition of the outer tread cap layer to, in one sense, provide a cushion for the outer tread cap layer.
For this invention, the specialized transition rubber layer is presented as a significant departure from said outer tread cap rubber layer, and said optional tread base rubber layer if used, in a sense that it contains at least one of scrap rubber and coal dust. In one embodiment, the tread cap rubber layer itself, and the optional tread base rubber layer if used, do not contain any appreciable amount of, and are preferably exclusive of, said scrap rubber and coal dust. In another embodiment, said tread cap layer rubber and/or optional tread base layer rubber may contain said scrap rubber and/or coal dust ingredients in a limited amount.
In this manner, then, the specialized transition tread rubber layer is considered herein to be neither of such tread cap rubber layer nor such optional tread base rubber layer because, in part, it contains a significant content of such less expensive rubber composition in a sense of containing lower grade ingredients as compared to the rubber compositions of said tread cap rubber layer and said optional tread base rubber layer.
In practice, as the tread cap rubber layer, and its associated tread lugs with their running surfaces, wears away during the running of the tire over time during the service of the tire, the underlying specialized transition rubber layer, which extends radially outwardly into a portion of the lugs, and optionally into the grooves, of the outer tread cap layer, becomes exposed and thereby becomes a new portion of the running surface of the tread prior to the tread being sufficiently worn to warrant removing the tire from service. In this manner, then, the scrap rubber and/or coal dust-containing underlying specialized transition rubber layer may present a new running surface for the tread after a sufficient amount of the outer tread cap rubber layer wears away. The lug and groove configuration of the worn tread is therefore substantially maintained, since the underlying specialized transition layer extends radially outward to include a portion of the tread lugs, and optionally the tread grooves of the tread cap layer, and rubber composition of the transition rubber layer presents a new running surface for the tread lugs.
In one embodiment then, such tire is provided wherein at a least a portion of said transition rubber layer is positioned within at least one of said tread lugs of said outer tread cap rubber layer in a manner to become a running surface of the tire upon at least a portion of said lug of said outer tread cap layer wearing away (e.g. as the tire is run in service) to expose said transition rubber layer.
Heretofore, various dual layered tire treads have been proposed which are composed of a cap/base construction in which the outer tread cap rubber layer contains a running surface for the tire and the underlying tread base rubber layer provides, in a sense, a cushion for the tread cap layer, such as for example U.S. Pat. No. 6,959,743 or of a dual tread base layer configuration, such as for example U.S. Pat. No. 6,095,217 as well as a cap/base construction in which the base layer extends into lugs of the tread and into its tread cap layer such as for example U.S. Pat. No. 6,336,486.
The tire tread of this invention differs significantly therefrom in that its transition rubber layer is intended to be exclusive of the tread cap rubber layer and the tread base rubber layer, if used, for example, in a sense that it contains at least one of partially depolymerized cured rubber, particulate pre-cured rubber and coal dust, and particularly by containing at least one of said depolymerized cured rubber and particulate pre-cured rubber.
In practice, scrap rubber in a form of partially depolymerized cured rubber (sometimes referred to as being recycle rubber) and in a form of particulate pre-cured rubber are well known to those having skill in such art.
Such recycle rubber is a vulcanized (cured) rubber which has been broken down by various processes, or combination of processes, which may include chemical breakdown, to form a partially depolymerized, and possibly partially devulcanized, rubber. Representative of such recycle rubber is, for example, partially depolymerized rubber as RNR 50B11™ from Rubber Resources.
Such particulate pre-cured rubber (sometimes referred to as ground cured rubber) may be obtained, for example, by physically grinding vulcanized rubber, which may include cryogenic grinding, to form small particles of cured rubber. Representative of such ground pre-cured rubber is, for example, TR-30™ from Edge Rubber.
It is important to appreciate that both the recycle rubber and the ground rubber relate to rubber which has been first vulcanized.
The resultant recycle rubber and ground rubber may have somewhat an appearance of unvulcanized rubber but have important differences and properties therefrom. They, particularly the recycle rubber, are composed of a mixture of polymer units of various and numerous constructions different from either unvulcanized or vulcanized rubber. Thus they are typically a complex mixture of largely unknown polymer(s), compounding ingredients, which may possibly contain small amounts of bits of textile fiber and the like.
In practice, it has been observed that, after adding sulfur and vulcanization accelerator curatives to recycle rubber and to ground cured rubber, followed by sulfur re-vulcanization thereof, the resulting physical properties, such as for example tensile, elongation and dynamic modulus, are usually somewhat lower than the corresponding properties of the original vulcanized rubber composition prior to its being partially depolymerized or ground into fine vulcanized rubber particles and resistance to abrasion is usually expected to be reduced more than most other physical properties of the cured rubber composition.
Coal dust is a carbonaceous dust from naturally occurring coal which significantly differs from synthetically produced rubber reinforcing carbon black composed of aggregates of primary carbon particles as would be well known to those having skill in such art. Coal dust might sometimes be referred to as coal fines. Coal dust is conventionally of significantly greater size (greater average diameter) than rubber reinforcing carbon black aggregates, is not rubber reinforcing in the sense of rubber reinforcing carbon black, represents a significantly lower cost filler than rubber reinforcing carbon black and, significantly, can be usually be used in greater quantities (concentration) in the rubber composition without significantly adversely affecting the processing of the rubber composition. Representative of such coal dust is, for example, Austin Black 325™ from Coal Fillers.
In the description of this invention, the terms “rubber” and “elastomer” where used herein, are used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, where used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients” and the term “compound” relates to a “rubber composition” unless otherwise indicated. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer, where such rubber is intended herein to not include said partially depolymerized pre-cured rubber and said particulate pre-cured rubber which are, instead, to be considered to herein as additives in a sense of being compounding ingredients for the rubber composition. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated. The term “Tg”, if used, means the middle point glass transition temperature of an elastomer determined by DSC (differential scanning calorimeter) at a heating rate of 10° C. per minute as would be understood by those having skill in such art.